The overall thrust of this project is the delineation of the molecular mechanisms involved in the physiological regulation of cholesterol biosynthesis in animals, including human subjects. The focus of our experimental work is the enzyme HMG-CoA reductase, which catalyzes the rate-limiting reaction of mammalian cholesterogenesis. In prior work supported by this grant, we described the in vitro interconversion of HMG-CoA reductase between active and inactive forms of widely differing catalytic activity. Conversion to the inactive form requires MgATP and one or more converter proteins. Conversion to the active form requires a different converter protein, but no cofactors. We have recently obtained preliminary evidence that the gamma phosphate of ATP is incorporated into reductase protein during inactivation. This suggests that the converter proteins may be a protein kinase and a protein phosphatase, respectively. We shall continue to study P32 labelling of reductase in an attempt to determine the validity of this hypothesis. In addition, we shall seek evidence for rapid changes in reductase activity in vivo attributable to the active-inactive interconversions that we observe in vitro. Work on rat and human leukocyte reductase described in prior notices has ceased with the departure of Dr. Nancy Young, who continues to work on these aspects of our original project in her laboratory at Cornell University Medical School in New York City.